In the field of security systems, intrusion into a secured area can be detected in a number of ways. Once an intruder is within the protected premises, acoustic and infrared sensors can be used to detect motion of the intruder, and to sound an alarm. However, it is desirable to sound an alarm at the time of the initial breach of the perimeter of the protected premises.
Various techniques are used to sound an alarm when an intruder enters a building by way of a protected access point. Such an access point might be a door or a window. Window break alarms include electrically conductive tape applied to the surface of the window to be protected, and the ends of the tape are connected to an electrical monitor which detects a breaking of the circuit, which may occur if the window is broken. This technique is effective, but may not be useful in those cases in which a window is left open, or in which application of the tape is inconvenient. Sound based window breakage alarms monitor the acoustic environment within a building, and react to sounds which approximate those associated with a window being broken. Such acoustic alarms need to maintain a memory of various possible acoustic signatures, and to filter the ambient noise looking for such signatures. This may be complex and costly.
One common way to protect access points on the perimeter of a building is by means of switches arranged to change state in the event that a window, door, or other access point is breached. A switch can be associated with a door or window to trigger an alarm if the door or window is opened.
In some cases, there is a possibility that a window may be broken out, rather than opened. This possibility is greatest with windows which are near ground level, such as basement windows, which are often casement types. Such windows are often protected by a "barrier bar," which is an elongated, spring-loaded, expansible bar or tube, similar to a curtain rod with a an internal spring. The barrier bar is cut to size, and assembled with the spring inside, and mounted across the inside of the aperture in which a window is located, and bearing against a switch. The switch is maintained in one of its two states by the pressure applied by the barrier bar. An electrical circuit monitors the state of the electrical switch, to sound an alarm or give a signal in the event that the state of the switch changes. An attempt to enter the protected premises by way of the window protected by the switch/barrier bar arrangement will, if all goes well, result in displacing the barrier bar, which removes the spring pressure holding the switch in its first state, and causing the switch to assume its second state, thereby sounding the alarm.
The barrier bar is somewhat effective, but suffers from the disadvantage of being timeconsuming to install, in that the barrier bar must be custom-sized to the window, which may require arriving at the site at which a barrier bar is to be installed with a full-length barrier bar, disassembling the barrier bar so as to be able to cut it to proper size, and then reassembling it. In use, the protection afforded by the barrier bar can be reduced if the person attempting entry knows of its existence. That person can simply apply sufficient pressure to the barrier bar in the direction of the switch to maintain the state of the switch, while moving the bar aside so as to enable entry.
Another type of protective arrangement for such windows is one in which a switch located adjacent the window to be protected is maintained in a given state by a washer or spacer located between pincers. A string or thread is attached to the washer or spacer, and strung across the window opening. A person attempting entry will, it is hoped, press on the thread, and thereby pull the spacer out of the switch, thereby sounding the alarm. This arrangement is somewhat easier to install than the barrier bar, but is subject to being defeated by simply cutting the thread.